Make sure to set your Garmin to NMEA output at 4800 baud (main-menu/setup/interface/nmea in nmea out).
The etrex venture is the same model as I was using when writing it.
I have no car_serial_rate= or notify= options at all, because those are player options and the last thing we want is the player to mess with the serial port.
Did you run hack_init so that the player is started with the -s- flag?
If you use hijack to force the empeg to DC mode you can also connect the gps directly to the serial port on the back of the player.